I. Field of the Invention
This invention relates to a method and apparatus for the impregnation of ceramic monolithic substrates and, more particularly, to such a method and apparatus which enable the impregnation of the substrate with a predetermined amount of catalyst in an efficient manner and without waste.
II. The Prior Art
The need to remove or convert the noxious components in vehicular exhaust gases is now well known as a means for overcoming air pollution. Also, the present and proposed future requirements for having catalytic exhaust gas converters on motor vehicles are quite well known. One form in which the catalyst for the converters is supplied is as catalytically coated rigid skeletal monolithic substrates, or honeycomb-type elements which are generally cylindrical or oval in shape, where there are a multiplicity of longitudinal passageways in each unit in order to provide a high surface area.
The rigid, monolithic, skeletal substrate structures are typically fabricated from ceramics which comprise refractory crystalline materials such as sillimanite, magnesium silicates, zircon, petalite, spodumene, cordierite, aluminosilicates, mullite, or combinations thereof. Such materials are generally considered to have a porous surface, but to improve the porosity of the surfaces of the skeletal surface, it is generally advisable to provide a highly porous alumina coating over the skeletal structure prior to effecting surface impregnation with a catalytically active material. These monolithic, substantially catalytically inactive skeletal substrate members have been described in prior art patents, as for example in Keith et al U.S. Pat. Nos. 3,331,787 and 3,565,830, such that it is not deemed necessary to describe them in detail herein.
Typically, and by way of example only, the catalytic component will comprise one or more of the noble and base metals and metal oxides of Groups IB, VB, VIIB, and VIII of the Periodic Table, particularly, copper, vanadium, chromium, manganese, iron, cobalt, nickel, platinum, palladium, rhodium, and ruthenium, with one catalytic metal being used singly or in combination with one or more other active metals.
While various methods are known in the art for coating a monolithic support with a refractory coating such as alumina and noble metal catalytic coatings such as platinum, palladium, and rhodium, such methods from the standpoint of cost are deficient in minimizing the amount of coating applied, especially when a costly catalytically active precious metal, e.g. platinum, palladium, or rhodium is codeposited with the high surface area refractory metal.
The instant disclosure reflects the results of continued efforts toward improving the techniques disclosed in copending application of Thomas Shimrock, et al, Ser. No. 596,993, filed Apr. 5, 1984 now U.S. Pat. No. 4,550,034, patented Oct. 29, 1985. That disclosure first presented the need in the art for precisely controlling the amount of alumina and metal catalyst slurries applied to ceramic monolithic catalyst substrates to reduce the amount of excess coating required so that there may result an improvement in efficiency of the process and a reduction in coating material loss.
That copending disclosure listed a number of prior patents and their deficiencies in achieving the goals sought by the present invention. Some additional prior patents should be considered which bear a relationship to the present disclosure. For example, U.S. Pat. No. 3,984,213 to Hoyer et al discloses a treating chamber for applying a coating slurry to a substrate. The slurry is introduced from the top and flows downwardly through the hollow substrate.
U.S. Pat. No. 4,038,939 to Hoyer et al discloses a process of impregnating a substrate by immersion within a treating chamber. It utilizes removal arm means which operate to effect a 90-degree turn of each substrate as it is removed from the treating chamber such that its honeycomb passageways are oriented in a generally horizontal manner to enable air blowing and drying of wet elements and to preclude slurry droplets from blocking the passageways.
According to U.S. Pat. No. 4,191,126 to Reed et al, slurry is applied to a substrate either by dip coating or by applying a coating charge to the upper end of the substrate. It is mentioned in the patent that it may be advantageous, after the substrate has been purged from one end, to invert the substrate and continue the purge from the opposite end.
In U.S. Pat. No. 4,384,014 to Young, there is a disclosure of impregnating a porous article placed on a base plate and covered with a cylinder sealingly mounted on the base plate in the manner of a bell jar. Vacuum is applied to the upper part of the cylinder and impregnant is admitted through the base plate under the action of the vacuum. When the process is completed, the vacuum is broken, excess impregnant is removed from the cover, and the cover is removed from the base plate.
However, as previously mentioned, none of these prior patents discloses any techniques for precisely controlling the amount of coating slurries applied to the substrate to thereby improve the efficiency of the process and reduce the coating material loss.